An acoustic characteristic common to most automotive sound systems is a rapid rate-of-change of the interaural phase difference (IPD) to a maximum of substantially 180 degrees, at substantially 200 Hz, occurring at both the left and right sides of the automotive listening environment. This IPD anomaly typically occupies a narrow “transition region” (e.g., 100 Hertz wide) and occurs in opposite polarities at the left and right sides of the listening environment. Additionally, this IPD anomaly may be characterized as an abrupt, non-linear phase shift as opposed to a linear phase shift or time delay function.
Therefore, it is desirable to provide a digital signal processing technique for symmetrical stereophonic image enhancement in an automotive listening environment. In particular, the technique introduces a rapid rate-of-change of phase shift between the signal channels, to a maximum of substantially 180 degrees, at substantially 200 Hertz. As a result, there exists two 180 degree phase shifts: one occurring naturally in the automotive listening environment and the other provided by the above-described digital signal processing technique. The two phase shifts occur within the narrow transition region, thereby correcting IPD to substantially zero above and below the frequency of the transition region. Since the transition region typically occupies less than 16% of the phase sensitive frequencies between 150 and 800 Hz, the effect of the non-corrected IPD in the transition region is generally not audible. In this way, the present invention provides theoretically optimal symmetrical IPD compensation for an automotive listening environment.